Optical tracking based on a change in optical reflection across a reference mark on an object to be tracked

ABSTRACT

An optical tracking device and method are disclosed for tracking lateral movement of an object. A scanning probe beam and a time-resolved detection are implement in the disclosed technique. A particular application is for tracking the eye movement during a laser surgery.

[0001] This application claims the benefit of U.S. provisionalapplication No. 60/083,248, filed on Apr. 27, 1998.

TECHNICAL FIELD

[0002] The present invention relates to tracking an object by opticalmeans, and more specifically, to automatic monitoring and tracking amovable object such as an eye.

BACKGROUND

[0003] Monitoring and tracking a laterally movable object are importantin many applications. In certain applications, it is desirable to have atracking device not only to monitor the displacement of the object butalso to follow the movement of the object without a significant delay.Tracking and following the eye movement during a laser eye surgery is anexample of such applications.

[0004] Many eye-tracking devices have been developed for eye surgerywith lasers, in particular, for photo-refractive surgery. A typicalphoto-refractive surgery scans an UV laser beam on the cornea tosculpture the profile of the corneal outer surface, one layer at a time.This procedure can correct various refractive disorders of the eye,including nearsightedness, farsightedness, and astigmatism.

[0005] Any eye movement during the surgery may adversely affect theoutcome of refractive correction. Immobilizing the eye movement during asurgery has been proven difficult in practice. A device automaticallytracking and compensating the eye movement is an attractive approach.For the nature of photo-refractive surgery, the tracking device needs tobe fast, accurate, and reliable.

[0006] U.S. Pat. No. 5,620,436 discloses use of a video camera tomonitor the eye's movement and to determine the position of an aimingbeam on the eye. U.S. Pat. No. 5,632,742 teaches projecting four laserspots on the eye and using a set of peak-and-hold circuits to determinethe position of the eye. In these designs, a ring shape reference isused for determining the eye position, and spatial stationary infraredbeams are applied to illuminate the reference. Sophisticated imagingsystem and electronics, such as a CCD camera or four peak-and-holdcircuits are implemented to determine the position of the reference. Thering shape references are practically either the limbus or the iris ofthe eye and the whole ring is needed as the reference for determiningthe eye position.

SUMMARY

[0007] Generally, any optically identifiable reference mark or indicatoraffix to an object can be used to indicate the position and movement ofthe object. The devices and methods disclosed herein apply an opticalprobe beam scanning repeatedly and rapidly over such a reference mark. Achange in the position of the reference mark can then be determined bymeasuring the change in the delay between a predetermined reference timeand the detected time at which the optical probe beam intercepts thereference mark. The reference mark can be artificially formed on theobject, or alternatively, can be an inherent mark on the object.

[0008] For the application of eye tracking, a reference mark may be thelimbus of the eye, which is the natural boundary between the transparentcornea and the white sclera. Optical scattering changes from one side ofthe limbus to the other significantly. Therefore the position of thelimbus can be detected by measuring the timing of the change in thescattered light of the probe beam as the probe beam scans across thelimbus. The devices and methods of the present disclosure will bedescribed by examples of eye tracking using a section of the limbus asthe reference mark.

[0009] In one embodiment, a section of the limbus is used as thereference mark and the x-y positions of the limbus are determined by twosets of linear positioning devices. The two linear positioning devicesare set for measurement along two mutually orthogonal axes.

[0010] Each linear positioning device consists of a scanning beamgenerator, a detection assembly, and a processing electronics. Thescanning-beam generator projects an infrared probe beam onto the eye andscans the probe beam across a section of the limbus repetitively. Thedetection assembly detects the infrared light scattered from the eye.The detected scattered-light signal is a time-resolved signal and has asequence of sharp steps corresponding to the probe beam repeatedlyacross the limbus. The timing of each sharp step depends on the limbusposition at the corresponding scan. The processing electronics convertsthe timing of the sharp steps into the positioning signal indicating theposition of the eye.

[0011] With the positioning signal, a system computer can then generatea control signal to steer the surgical laser beam to follow the movementof the eye. Hence, accurate laser surgery can be achieved even thoughthe eye may move during the surgery.

[0012] In this embodiment, about a quart of the limbus is used todetermine the x and y positions of the eye. This is particularlyimportant for a new type of refractive surgery so called LASIK, in whichpart of the limbus is obstructed during the surgery. This embodiment canuse the limbus section that is not blocked and thus it can use thelimbus as a reliable reference mark for LASIK.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013]FIG. 1 is a schematic diagram showing one embodiment of anopen-loop optical monitoring and tracking system;

[0014]FIG. 1a shows timing diagrams of the scattered-light signal fromthe eye and the reference signal generated by a scanning beam generator.

[0015]FIG. 2 is a schematic diagram showing an embodiment of aclose-loop optical monitoring and tracking system;

[0016]FIG. 3a is a schematic diagram showing one embodiment of ascanning-beam generator;

[0017]FIG. 3b is a schematic diagram showing another embodiment of ascanning-beam generator;

[0018]FIG. 3c is a schematic diagram showing a third embodiment of ascanning-beam generator;

[0019]FIG. 4 is a block diagram showing a processing electronics for theoptical monitoring and tracking systems of FIGS. 1 and 2;

[0020]FIG. 5a is a schematic diagram illustrating simultaneous trackingof an eye in two different directions by two scanning probe beamsprojected on the limbus.

[0021]FIG. 5b shows two scanning probe beams projected on a partiallyobscured limbus to track the eye movement in two different directions ina LASIK surgery.

DETAILED DESCRIPTION

[0022]FIG. 1 shows a schematic diagram of one embodiment of an opticalmonitoring and tracking system 100 for an eye 10. The system 100implements an open loop configuration that includes a position sensingmodule 101, a system computer 80, and a beam steering module 60 (e.g., ax-y scanner). The position-sensing module 101 projects a scanning probebeam 4 and monitors the position of the eye 10. The system computer 80controls the beam steering module 60 to guide a surgical laser beam 62to a desired position on the eye 10. As an open loop configuration, thescanning probe beam 4 dose not follow the movement of the eye 10 andonly one beam steering module 60 is required.

[0023] For illustration purpose, the position-sensing module 101 shownin FIG. 1 is only a linear positioning device and is for monitoringone-dimensional eye movement only (e.g., along x-direction). Todetermine the eye's movement in two dimensions, a second set of linearpositioning device is needed to monitor the movement of the eye 10 alonga second different direction, e.g., the y-direction orthogonal to thex-direction.

[0024] The position-sensing module 101 comprises a scanning beamgenerator 30, a collection lens 6, a photo-detector 7, and a processingelectronics 50. The limbus 11 of the eye 10 is used as a reference mark20. The scanning-beam generator 30 projects a scanning probe beam 4across the reference mark 20. The scanning probe beam 4 may repeatedlystart from a fixed point and is scanned at a constant speed over apredetermined tracking range. The scanning-beam generator 30 alsoproduces a reference signal 31 to indicate a reference point of thescanning.

[0025] The lens 6 is disposed at a proper position relative to the eye10 to collect the scattered light 5. The photo-detector 7 receives andconverts the scattered light 5 into an electrical signal, i.e., thescattered-light signal 8. The scattering from the sclera side 13 of theeye 10 is approximately 20 times stronger than that from the transparentcornea side 14. Hence, the intensity of the scattered light 5 exhibits asignificant change when the probe beam 4 scans across the limbus 11.This intensity change of the scattered light 5, in turn, generates asharp step in the scattered-light signal 8. The timing of this sharpstep depends on the position of the eye 10.

[0026] In one implementation, an infrared laser beam (at 830 nm) ofabout 100 μW is used as the scanning probe beam 4 and the collectionlens 6 having an aperture of about 18 mm is located about 30 cm awayfrom the eye 10. Detector 7 receives a scattered-light power of about 20nW when the probe beam 4 is on the sclera side.

[0027]FIG. 1a shows timing diagrams of the scattered-light signal 8 andthe reference signal 31. The scattered-light signal 8 has a sequence ofsharp steps and each sharp step 9 corresponds to a scan of the probebeam 4 across the limbus 11. The sharp step 9 has a time delay Td withrespect to the reference point 31 s of the scanning. This time delay Tddepends on the position of the limbus 11 and varies as the eye 10 moves.The processing electronics 50, which may include a microprocessor,processes the reference signal 31 and the scattered-light signal 8 todetermine this time delay Td for each scan. This time delay Td is thenused to determine the position of the limbus 11. The lines Vth representthe threshold voltage for triggering.

[0028] To operate the tracking device 100, an initial time delay Td₀ oreye position is first registered and stored in the system computer 80.The time delay Td₀ of subsequent scans is then compared with the initialtime delay Td₀ to calculate a displacement of the eye 10. With thiscalculated displacement, the system computer 80 can generate a controlsignal 81 to drive the beam steering module 60 to steer the surgicallaser beam 62 to follow the movement of the eye 10.

[0029] As an open loop device, the scanning probe beam 4 does not movewith the eye 10. The beam steering module 60 can be used simultaneouslyto compensate the eye movement and to scan the surgical laser beam 62 onthe eye 10. In this case, the control signal 81 may consist of ascanning signal and an offset signal. The scanning signal scans thesurgical laser beam 62 in a predetermined pattern while the offsetsignal offsets the scanning to compensate for the eye movement. Thisopen-loop device is relatively simple and is good for tracking smallmovement of the eye 10.

[0030]FIG. 2 shows a schematic diagram of a close-loop tracking device200. In the close-loop configuration, both the scanning beam 4 and thesurgical beam 62 are steered to the eye 10 by a common steering module60. Consequently, both the scanning probe beam 4 and the surgical laserbeam 62 follow the movement of the eye 10.

[0031] In implementation, the scanning probe beam 4 is directed into thebeam steering module 60 and reflected onto the reference mark 20 (i.e.the limbus 11). A dichromatic mirror 70 is placed in the path of thescanning probe beam 4 to couple the surgical laser beam 62 into the beamsteering module 60. The dichromatic mirror 70 reflects light at thewavelength of the surgical laser beam 62 but transmits light at thewavelength of the scanning probe beam 4. The surgical laser beam 62 isreflected from the beam steering module 60 and projected onto the eye10.

[0032] Again, the scattered light 5 from the reference mark 20 iscollected by a lens 6 and detected by a photo-detector 7, which producesan output of scattered-light signal 8. Similar to the open loop device100, the scatted-light signal 8 has a sharp step 9 corresponding to eachscan of the probe beam 4 across the boundary of the reference mark 20.The sharp step 9 has a time delay Td with respect to the reference point31 s of corresponding scan. A processing electronics 50 determines thistime delay Td for each scan.

[0033] To operate the tracking device 200, an initial time delay Td₀ oreye position is first registered and stored by the system computer 80.The time delay Td of later scans is then compared with the initial timedelay Td₀. Any deviation of Td from Td₀ is used as an error signal todrive the beam steering module 60 such that to bring the error signaltoward zero. Through this process, the beam steering module 60 deflectsthe scanning probe beam 4 to follow the movement of the eye 10. Seeingthe same deflection as the scanning probe beam 4, the surgical laserbeam 62 can thus impinge on any predetermined position of the eye 10 asif the eye remains stationary.

[0034] As a close loop device, the relative position between the traceof the scanning probe beam 4 and the reference mark 20 is kept constantduring the operation. The beam steering module 60 is thus used solelyfor compensating the eye movement. A second beam steering module 90 isrequired to scan the surgical laser beam 62 on the eye 10 for surgerypurpose. In this case, the control signal 81 to beam steering module 60is simply the driving signal to compensate the eye movement. The controlsignal 82 to beam steering module 90 is simply the programmable signalto scan the surgical laser beam 62. The close loop device 200 isrelatively more complicate but it can track a relative largedisplacement of the eye 10.

[0035]FIG. 3a shows one embodiment of a scanning-beam generator 30 athat produces a scanning probe beam 4 a. The generator 30 a includes aninfrared-light source 32 a, which produces an infrared-light beam 33 aprojected onto a rotating blade 35 a. The blade 35 a has a set ofpinholes 36 a evenly distributed on a circle. A motor 34 a drives theblade 35 a at a constant rotation speed. The pinholes 36 a are thusscanned across the infrared-light beam 33 a at a constant speed.

[0036] A lens 37 a focuses onto a reference ring 20 (i.e. the referencemark) the infrared-light beam 38 a that is transmitted through thepinhole 36 a. As the pinhole 36 a is scanned across the infrared beam 33a, the image of the pinhole 36 a is scanned across the reference ring20. Thus, the transmitted infrared beam 38 a may serve as the scanningprobe beam 4 of FIG. 1.

[0037] A beam splitter 39 a directs a small portion of the beam 38 aonto a reference photo-detector 40 a. This reference photo-detector 40 ahas a tiny light-sensitive area and the detected signal is thus asequence of spikes as the split beam scans across the reference detectorrepetitively. The output signal from the photo-detector 40 a defines areference point of the scanning and serves as the reference signal 31 ofFIG. 1.

[0038] In this embodiment, the infrared-light source 32 a can be simplya light emitted diode. The repetition rate of the scanning probe beam 4can be up to the kilohertz range. For example, the motor 34 a may run at100 rotation per second and the blade 35 a may have 10 pinholes 36 a onit.

[0039]FIG. 3b shows another embodiment of a scanning-beam generator 30 bproducing a scanning probe beam 4. The generator 30 b includes aninfrared-light source 32 b, which produces an infrared-light beam 33 bdirected onto a disk 35 b. The disk 35 b holds a set of identical lenses36 b evenly distributed on a circle. A motor 34 b rotates the disk 35 band the lenses 36 b are scanned across the infrared-light beam 33 b at aconstant speed.

[0040] The infrared-light beam 38 b transmitted through a lens 36 b isfocused onto a reference ring 20. As the lens 36 b is scanned across theinfrared-light beam 33 b, the focused beam 38 b is scanned across thereference ring 20. Thus, the focused infrared-light beam 38 b may serveas the scanning probe beam 4 of FIG. 1.

[0041] Again, a beam splitter 39 b directs a small portion of the beam38 b onto a reference photo-detector 40 b. The output signal from thephoto-detector 40 b defines a reference point of the scanning and servesas the reference signal 31 of FIG. 1. In this embodiment, theinfrared-light source 32 b is preferably either a pre-focused beam or apoint source.

[0042]FIG. 3c is a schematic diagram showing a third embodiment of ascanning-beam generator 30 c producing a scanning probe beam 4. Thegenerator 30 c includes an infrared-light source 32 c, which produces aninfrared-light beam 33 c directed into a lens 37 c. The transmittedinfrared beam 38 c is reflected by a mirror 36 c and focused onto areference ring 20. The mirror 36 c is driven by a scanner head 34 c toscan the infrared beam 38 c across the reference ring 20. Thus, thetransmitted infrared beam 38 a may serve as the scanning infrared beam 4of FIG. 1.

[0043] Similarly, a beam splitter 39 c directs a small portion of thebeam 38 c onto a reference photo-detector 40 c. The output signal fromthe photo-detector 40 c defines the reference point of the scanning andserves as the reference signal 31 of FIG. 1. The scanner 34 c scans thebeam 38 c back and forth. A synchronized signal from the scanner 34 ccan also be used as a reference point of the scanning. In thisembodiment, the infrared-light source 32 c can be either a collimatedbeam or a point source.

[0044]FIG. 4 is a block diagram showing one embodiment of the processingelectronics 50. This processing electronics 50 includes a first triggercircuit 52, a second trigger circuit 54, and a microprocessor 58. Thereference signal 31 from the scanning beam generator 30 is fed into thefirst trigger circuit 52 to produce a TTL output signal 53 carrying thetiming of the reference signal 31. The scattered-light signal 8 from thephoto-detector 7 is fed into the second trigger circuit 54 to produce aTTL output signal 55 carrying the timing of the scattered-light signal8.

[0045] The microprocessor 58 reads in the signal 53 and signal 55 tocalculate a time delay Td between the two signals. This time delay Tdindicates the relative position of the reference mark 20 to the scanningprobe beam 4. This delay Td can be compared with an initial delay Td₀registered and stored by the system computer 80 at the very beginning ofthe tracking.

[0046] For an open loop device 100, any change of the delay Td from itsinitial value Td₀ can be used to determine a displacement of the eye 10from its initial position. The determined displacement can then beconverted into an offset signal combined in the control signal 81 todeflect the surgical laser beam 62 to follow the movement of the eye 10.

[0047] For a close loop device 200, any deviation of the delay Td fromits initial value Td₀ is used as an error signal to drive the beamsteering module 60 such that to bring the error signal toward zero. Thebeam steering module 60 thus deflects both of the scanning probe beam 4and the surgical laser beam 62 to follow the movement of the eye 10.

[0048] The above-described operation of the processing electronics 50 isrepetitively for every scan of the probe beam 4. The first triggercircuit 52 and the second trigger circuit 54 should be resetautomatically after the signal 53 and signal 55 are read by themicroprocessor 58.

[0049] The processing electronics 50 shown in FIG. 4 is for one axistracking. To track the two-dimensional movement of the eye 10, anotherpair of the trigger circuit should be used.

[0050]FIG. 5a shows schematically two scanning probe beams 4 x and 4 yprojected on a reference ring 20 (the limbus 11) for two-dimensionpositioning detection. The two scanning probe beams 4 x and 4 y are setalong two approximately perpendicular directions and occupy about onequart of the limbus 11.

[0051]FIG. 5b shows how the tracking device remains full performance forLASIK. In a LASIK surgery, a disk shape flap is laminated from thecornea and about one quart of the perimeter is uncut to maintain theflap attached to the cornea. The flap is folded over during the surgeryto allow laser ablation on the corneal bed. The folded flap 15 coversabout one third of the limbus 11 and may disable those eye trackingdevices which rely on the whole limbus as the reference. The corneal bedafter the flap is folded becomes less smooth and the scattered lightfrom the corneal bed may disturb those tracking devices that use thepupil as a reference.

[0052] As illustrated in FIG. 5b, the two scanning beams 4 x and 4 y useonly the limbus section that is not covered by the cornea flap 15.Therefore, the limbus 11 remains as a good reference for the trackingdevice of the present invention.

[0053] In all the above description, the tracking device is to steer asurgical laser beam 62 to follow the eye movement. Obviously, the sametracking mechanism can guide any other light beam or simply an opticalpath to follow the eye movement. Therefore, the above technique can beused to other surgical or diagnosis application in which compensatingthe eye movement is desirable.

[0054] Although the above embodiments are described with a specificreference to eye tracking, the techniques can be generally used to tracklateral movement of other object with an optical reference mark. Variousmodifications can be made without departing from the scopes of theappended claims.

What is claimed is:
 1. A method for optically tracking movement of anobject, comprising: selecting a reference mark on an object where firstand second surface areas on opposite sides of the reference mark on theobject are different in an optical property; repetitively scanning anoptical probe beam from the first surface area to the second surfacearea across the reference mark at a selected scanning speed along aselected direction so that a property of a scattered probe beam that isscattered from the object changes due to the change in the opticalproperty in the first and second surface areas when the probe beampasses through the reference mark; detecting a time at which the changein the property of the scattered probe beam occurs when the probe beamscans across the reference mark; determining a time difference betweenthe time and a reference time; and using the time difference todetermine an amount of movement of the object along the selecteddirection.
 2. The method as in claim 1 , wherein the reference markincludes a boundary between the first and second surface areas withdifferent optical reflectivities, and the property in the scatteredprobe beam is an optical amplitude.
 3. The method as in claim 2 ,wherein the object is an eye and the reference mark includes the limbusof the eye with the sclera area being the first surface area and thecornea area being the second surface area.
 4. The method as in claim 1 ,further comprising: using the measured movement of the object along theselected direction to control a direction of another optical beamincident to the object to follow the movement of the object.
 5. Themethod as in claim 4 , further comprising: scanning the other opticalbeam over the object according to a predetermined spatial pattern inaddition to directing the other optical beam to track the object.
 6. Themethod as in claim 1 , further comprising: selecting a second referencemark located between the first and second surface areas and orientatedin a direction different from the reference mark; repetitively scanninga second optical probe beam from the first surface area to the secondsurface area across the second reference mark at a second selectedscanning speed along a second selected direction so that a property of asecond scattered probe beam that is scattered from the object changesdue to the change in the optical property in the first and secondsurface areas when the second probe beam passes through the secondreference mark, wherein the second selected direction is different fromthe selected direction; detecting a time at which the change in theproperty of the second scattered probe beam occurs when the second probebeam scans across the second reference mark; determining a timedifference between the time and a second reference time; using the timedifference to determine an amount of movement of the object along thesecond selected direction; and determining movement of the object in asurface defined by the selected direction and the second selecteddirection.
 7. The method as in claim 6 , wherein the second selecteddirection is substantially orthogonal to the selected direction.
 8. Themethod as in claim 6 , further comprising: using the measured movementof the object along the selected direction and the second selecteddirection to control a direction of another optical beam incident to theobject to follow the movement of the object.
 9. The method as in claim 8, further comprising: scanning the other optical beam over the objectaccording to a predetermined spatial pattern in addition to directingthe other optical beam to track the object.
 10. The method as in claim 6, wherein the scanning of the probe beam is independent of the movementof the object.
 11. The method as in claim 6 , wherein the scanning ofthe probe beam is controlled to follow the movement of the object. 12.The method as in claim 1 , wherein the scanning of the probe beam isindependent of the movement of the object.
 13. The method as in claim 1, wherein the scanning of the probe beam is controlled to follow themovement of the object.
 14. An optical device, comprising: a probescanner to scan an optical probe beam across a reference mark on anobject where first and second surface areas on opposite sides of thereference mark on the object are different in an optical property, saidprobe scanner operable to repetitively scan the optical probe beam fromthe first surface area to the second surface area across the referencemark at a selected scanning speed along a selected direction; and aprobe detection unit positioned to receive a scattered probe beam thatis scattered from the object and to detect a change in a property of thescattered probe beam due to the change in the optical property in thefirst and second surface areas when the probe beam passes through thereference mark, wherein said probe detection unit is operable to detecta time at which the change in the property of the scattered probe beamoccurs and an amount of movement of the object along the selecteddirection according to a time difference between the time and areference time.
 15. The device as in claim 14 , further comprising: anoptical scanner to receive and scan an optical beam over the objectaccording to a selected spatial scanning pattern; and a control unitoperable to control the optical scanner to track the movement of theobject according to the detected amount of movement of the object,wherein scanning of said probe scanner is independent of the movement ofthe object.
 16. The device as in claim 15 , wherein the optical beam isoperable to interact with the surface of the object so as to change ashape of the object and the probe beam does not change the shape of theobject.
 17. The device as in claim 14 , further comprising: an opticalscanner to receive and scan an optical beam over the object according toa selected spatial scanning pattern; a shared optical scanner positionedto receive both said optical beam from said optical scanner and saidoptical probe beam from said probe scanner and to direct both saidoptical beam and said probe beam to the object; and a control unitoperable to control the shared optical scanner to control said opticalbeam and said probe beam to track the movement of the object accordingto the detected amount of movement of the object.
 18. The device as inclaim 14 , wherein said probe scanner includes a probe light source toproduce the probe beam, a projecting lens to project the probe beam, anda rotating wheel having a plurality of apertures located between theprojecting lens and the probe light source.
 19. The device as in claim14 , wherein said probe scanner includes a probe a probe light source toproduce the probe beam, and a rotating wheel having a plurality ofprojecting lenses to rotate said projecting lenses into an optical pathof the probe beam, one at a time.
 20. The device as in claim 14 ,further comprising: a second probe scanner to scan a second opticalprobe beam across a second reference mark located between the first andsecond surface areas and orientated in a direction different from thereference mark, said second probe scanner operable to repetitively scanthe second probe beam at a second selected scanning speed across thesecond reference mark along a second selected direction, wherein asecond scattered probe beam produced from scattering of the second probebeam from the object is detected to determine an amount of movement ofthe object along the second selected direction.